An economical retroreflecting base material comprises substantially a monolayer of glass microspheres embedded in a polymeric binder layer, a specularly reflective layer covering the polymeric layer, and a pressure-sensitive adhesive layer by which the retroreflecting base material can be mounted onto a substrate such as a license plate blank. This composite is then dipped into a solution of resin which is allowed to harden to provide a transparent, weather-resistant cover film which brings the specularly reflective layer into proper focus. Although this dipping and hardening process is slow and labor-intensive, it is often used where labor costs are low, e.g., in prison industries.
When higher retroreflective brightness is desired, the retroreflecting base material may be made as illustrated in FIG. 6 of U.S. Pat. No. 4,511,210 (Tung et al.), which base material includes a monolayer of glass microspheres 21, a spacing layer 23, a specularly reflective layer 24, and a pressure-sensitive adhesive layer 25. After adhering this to a substrate, the composite may be dipped into a solution of resin to provide a transparent cover film as described above and mentioned in Example 4 of the Tung patent. Much faster production rates are realized if the cover film is preformed as in Example 1 of the Tung patent. Also, the use of a preformed transparent cover film avoids the pollution that would be created upon drying a solution. The cover film of that Example 1 is polymethylmethacrylate which functions well when the retroreflective sheeting has a rigid, flat support such as a highway sign. However, that material is not sufficiently extensible to withstand the stretching encountered in the embossing of a license plate or the application to irregular surfaces. Another preformed transparent cover film that has been used is biaxially oriented poly(ethylene terephthalate) film, but its high strength has inhibited its use in retroreflective sheeting which is to undergo stretching as in the embossing of a license plate. Also, those preformed transparent cover films have required a second pressure-sensitive adhesive layer, the need for which tends to defeat the economy of the retroreflective sheeting. Furthermore, the second pressure-sensitive adhesive layer lies in the optical path and may degrade, thus reducing the optical efficiency.
Because of the enclosed-lens nature of the above-described retroreflective sheetings, incident light rays are focused onto the specularly reflective layer irrespective of whether the front of the sheeting is wet or dry. This capability was first taught in U.S. Pat. No. 2,407,680 (Palmquist et al.), which discloses retroreflective sheeting that has been sold commercially for many years in large volume and to the general satisfaction of its users. In making retroreflective sheeting of the Palmquist patent, a transparent cover film is coated from solution, typically a thermoset resin such as an alkyd resin or an acrylic resin. However, to permit the retroreflective sheeting to be embossed, the cover film has been a thermoplastic resin such as plasticized polyvinyl chloride coated from solution.